The present invention relates to oximetry sensors and, in particular, pulse oximetry sensors which include coded information relating to characteristics of the sensor.
Pulse oximetry is typically used to measure various blood flow characteristics including, but not limited to, the blood-oxygen saturation of hemoglobin in arterial blood and heart rate of a patient. Measurement of these characteristics has been accomplished by use of a non-invasive sensor which passes light through a portion of the patient's tissue where blood perfuses the tissue, and photoelectrically senses the absorption of light in such tissue. The amount of light absorbed is then used to calculate arterial blood oxygen saturation.
The light passed through the tissue is selected to be of one or more wavelengths that are absorbed by the blood in an amount representative of the amount of the blood oxygenation. The amount of transmitted light passed through the tissue will vary in accordance with blood oxygenation. For measuring blood oxygen level, such sensors have been provided with light sources and photodetectors that are adapted to operate at two different wavelengths, in accordance with known techniques for measuring blood oxygen saturation.
An example of an encoding mechanism is shown in U.S. Pat. No. 4,700,708, incorporated herein by reference. This relates to an optical oximeter probe which uses a pair of light emitting diodes (LEDs) to direct light through blood-perfused tissue, with a detector picking up light which has not been absorbed. The operation depends upon knowing the wavelengths of the LEDs. Since the wavelengths of LEDs actually manufactured can vary, a coding resistor is placed in the sensor with the value of the resistor corresponding to the actual wavelength of at least one of the LEDs. When an oximeter is turned on, it first applies a current to the coding resistor and measures the voltage to determine the value of the resistor and thus the value of the wavelength of the LED(s) in the probe.
U.S. Pat. No. 5,259,381, incorporated herein by reference, recognizes that the coded value of the wavelength of the red LED provided by a coding resistor may be inaccurate, since the actual wavelength can vary with temperature. Accordingly, this patent teaches including a temperature sensor in the oximeter sensor to measure the actual temperature. With the actual temperature, and the coded wavelength value, a look-up table can be consulted to determine the actual LED wavelength for that temperature.
Another method of storing coded information regarding the characteristics of the LEDs is shown in U.S. Pat. No. 4,942,877, incorporated herein by reference. This patent discloses using an EPROM memory to store digital information, which can be provided in parallel or serially from the sensor probe to the remote oximeter.
Other examples of coding sensor characteristics exist in other areas. In U.S. Pat. No. 4,446,715, incorporated herein by reference, and assigned to Camino Laboratories, Inc., a number of resistors are used to provide coded information regarding the characteristics of a pressure transducer. U.S. Pat. No. 3,790,910, incorporated herein by reference, discloses another pressure transducer with a ROM storing characteristics of the individual transducer. U.S. Pat. No. 4,303,984, incorporated herein by reference, shows another sensor with digital characterization information stored in a PROM, which is read serially using a shift register.
Typically, the coding element is mounted in the sensor itself. For instance, U.S. Pat. No. 4,621,643, incorporated herein by reference, shows the coding resistor mounted in the sensor element itself. In addition, U.S. Pat. No. 5,246,003, incorporated herein by reference, shows the coding resistor being formed with a printed conductive material on the sensor itself.
In some devices, an electrical connector coupled by a cable to a device attached to a patient may include a coding element. For example, U.S. Pat. No. 3,720,199, incorporated herein by reference, shows an intra-aortic balloon catheter with a connector between the catheter and the console. The connector includes a resistor with a value chosen to reflect the volumetric displacement of the particular balloon. U.S. Pat. No. 4,684,245, incorporated herein by reference, discloses a fiberoptic catheter with a module between the fiberoptic and electrical wires connected to a processor. The module converts the light signals into electrical signals, and includes a memory storing calibration signals so the module and catheter can be disconnected from the processor and used with a different processor without requiring a recalibration.
A method for tuning the wavelength, and also providing a coded value in parallel with the LEDs, is shown in U.S. Pat. No. 5,758,644, incorporated herein by reference, and assigned to Masimo Corporation. This patent shows a way to reduce the number of leads necessary for the sensor. Claim 21 of the '644 patent also discusses a information element in parallel with the photosensing element, which is read at a predetermined voltage. Although the '644 patent does not describe how this is read for the photosensor, the reading of a coding resistor in parallel with the LED(s) is described as being done by using a lower level voltage below the shoulder of the exponential curve, as described in col. 18, lines 30-67.